15+18 is 33. There are many solutions to this question, but I do not know which solutions are shown/given to you. I will wing it.
3(5+6) can also be written as 3 x (5+6) or in a more simpler form, 5+6= is 11. So when we multiply 11 x 3, we get 33 which is also equal to 3(5+6) = 33. Hope this helps.